Really depends how far U.S./NATO wants to escalate.
People said the same thing just a few months ago that Russia's build up was just a scare tactic and there won't be a war in Ukraine, and here we are.
If the U.S./NATO continues to supply weaponry to Ukraine, especially going beyond small arms to fighter jets, S-300s, and Russia is taking very significant losses a few months down the road to a point where its no longer able to defeat Ukraine, IMO Russia will (and should) retaliate with tactical nukes.
IMO the current Russian leadership has the balls to use nukes and Putin's threats aren't empty. Its going to get real interesting if Russia retaliates with a tactical nuke because what is the U.S./NATO going to do then? They have no more sanctions to use. Are they going to nuke Russia back to escalate further? Is the U.S. and EU willing to trade cities with Russia?
If Putin nukes anybody for anything short of a major attack on Russia itself he's a f--king idiot. Why? Because at that point he's basically committing suicide.